<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
    <div style="display: flex;width: 1000px;height: 500px">
        <div style="width: 300px;height: 500px;">
            <a href="1.html" target="myiframe">1.html</a><br>
            <a href="2.html" target="myiframe">2.html</a><br>
        </div>
        <iframe name="myiframe" width="700" height="500"></iframe>
    </div>
</body>
</html>